In the art, methods such as BART (“Bandwidth Available in Real Time”) and TOPP (“Trains of Packet Pairs”) have been developed for estimating capacity in a network and sends a sequence of packets, also known as a probe train, from a transmitting node to a receiving node. The probe train is sent at a specific rate depending on packet size and time interval between the packets in the train. If the send rate of the train is greater than the available path capacity (APC) of the network path, there will be a transient congestion at a bottleneck link. Due to the congestion, the probe-train packets will be buffered in the node just before the bottleneck link which will spread out the packets in time. The probe train will therefore be received by the receiving node at a lower rate than the send rate.
The difference between the preset send rate and the actual receive rate is used for estimating APC and tight link capacity (TLC). In BART and TOPP, the send rate is changed for each train within a specified interval.
Normally, the packet size is held constant during a measurement session. The send rate can then be varied by varying the time interval between the packets. This packet interval is typically measured from the start of a packet to the start of next packet at the transmitting node and from the end of a packet to the end of next packet at the receiving node. The packet interval at the transmitting is illustrated in FIG. 2, where N denotes the total number of packets in the train.
The relations are:    send_time_between_packets=bits_per_packet/send_rate    receive_time_between_packets=bits_per_packet/receive_rate
In a network with a TLC=100 Mbps, for instance a network with fast Ethernet links, a typical send rate interval is 40-150 Mbps. Using a packet size of 1500 Byte (12000 bits) these send rates translate to:    40 Mbps: send_time_between_packets=12000 bit/40 Mbps=300 us    150 Mbps: send_time_between_packets=12000 bit/150 Mbps=80 us
The receive times will be slightly longer depending on the amount of cross traffic, but not shorter than 12000/100 Mbps=120 us, due to the capacity constraint of the network.
In order to be able to transmit at the maximum send rate of 150 Mbps, the resolution of a clock for the probe trains should be 80 us.
However, many systems have clocks with a resolution in the range of 1 ms. For instance, a mobile phone has a clock which is considerably faster than 1 ms, but this clock handles numerous tasks of the phone, and the operating system is not likely to update a task for estimating transmission capacity more often than once every millisecond. These systems will not be able to accomplish send rates above 12000 bit/0.001 s=12 Mbps, which means that with the current methods they will not be able to perform capacity estimates in networks with higher capacities than about 10 Mbps. Further, the possible send rates are limited by 1 ms steps of the timer, e.g. 12000/0.002=6 Mbps, 12000/0.003=4 Mbps, 12000/0.004=3 Mbps. In high-capacity systems, such as for example Long Term Evolution (LTE) systems, monitoring of utilization of the system capacity is important.